Stacking Interactions between Square-Planar Metal Complexes with 2,2′-Bipyridine Ligands. Analysis of Crystal Structures and Quantum Chemical Calculations

Stacking interactions between square-planar metal complexes containing bipyridine ligands (bipy) were studied by analyzing data in the Cambridge Structural Database (CSD) and by density functional theory (DFT) calculations. In most of the crystal structures, two bipy complexes were head-to-tail oriented. On the basis of the data from CSD, we classified the overlaps of bipy complexes into six types. The types were defined by values of geometrical parameters, and the interactions of the same type have very similar overlap geometries. The most frequent are the structures with quite large overlap area including chelate rings and pyridine fragments. The overlap is often influenced by ligands coordinated at the third and fourth coordinating positions or by molecules (ions) from the environment in the crystal structure. The interaction energies of all types of overlap were calculated on model systems using the DFT (TPSS-D3) method. The strongest calculated interaction has an energy of −31.66 kcal/mol and large area of overlap. By decreasing the overlap area, the strength of interactions decreases. The weakest calculated interaction has an energy of −7.26 kcal/mol and the small overlap area of pyridine fragments. These results presenting the geometries and energies of stacking interactions can be very important for various molecular systems.